1. Field of the Invention
This invention relates to extended residual, controlled release fertilizers exhibiting diffusion release kinetics and to processes for the preparation thereof.
2. Description of Related Art
Controlled release fertilizer technologies (also known as "slow release" fertilizers) can be categorized into various groups with one of such groups being coated fertilizers. Of the coated fertilizers, sulfur coated urea (SCU) has received the most extensive use and commercial development in view of its favorable economics. Another group of coated fertilizers relates to polymer or resin coated fertilizers which have shown superior longevity to sulfur coated products but have been restricted in use and commercial application as a result of their high relative cost of production predominantly due to the cost of the polymeric material, the use of recovery systems in their production and other processing considerations.
Accordingly, it has been a continuing problem to provide economically advantageous sulfur coated fertilizer products having enhanced release characteristics including extended residual properties such that the sulfur coated products provide release results approximating resin coated fertilizers at lower cost.
Sulfur coated ureas were first developed in 1968 by the Tennessee Valley Authority (TVA) in Muscle Shoals, Ala., as an economical system for reducing the rate of dissolution of urea particles when they are applied to the soil as a fertilizer. As a result of its early work, TVA determined the necessity for applying a sealant material in order to fill in the imperfections which occur in the sulfur coating as the sulfur cools. U.S. Pat. Nos. 3,295,950 and 3,342,577 describe the TVA sulfur coating process and also the sealant material. In addition, TVA's Bulletin Y-79 (1974) describes the development of a number of sealants over the years including TVA's original SAE 30 oil and Shellmax 800 microcrystalline wax. The current TVA recommended sealant which is now used by the majority of manufacturers of sulfur coated urea is described in TVA Bulletin Y-181 (1983). This sealant is a mixture of 30% (by weight) low molecular weight polyethylene resin in 70% (by weight) brightstock mineral oil.
The development of TVA's sulfur coated urea technology is further detailed in Bulletin Y-79, National Fertilizer Development Center, Tennessee Valley Authority, Muscle Shoals, Ala., August, 1974. In that bulletin, process conditions were described for urea preheating, sulfur coating, mixing of wax and microbicide and application to SCU in a second drum, cooling of the topcoated product and addition of a conditioner in a third drum.
The TVA process is further described in TVA Bulletin Y-136 (1978). Furthermore, the requirement for a sealant for sulfur coated urea (SCU) has been documented in the literature.
The mechanism of nutrient release from sulfur coated urea has been the subject of considerable study. However, it is now generally accepted that sulfur coated ureas release through pores and fissures in the sulfur coating. In this regard, Jarrell, W. (1977) Nitrogen Release from Granules of Sulfur-Coated Urea, Oregon State University, Ph.D., 1977, p. 57, reported that microorganisms colonize the surface of the sulfur coated products and degrade the coating at a rate dependent on the growth rate of the colony and the activity of the organisms. No urea is released to the environment until pin holes and cracks in the sulfur are exposed by the degradation of the wax coating.
As applied in the field, the nutrient release of such fertilizers has been found to be non-uniform and temperature sensitive due to variation in microbial activity. TVA's U.S. Pat. No. 3,342,577 to G. M. Blouin and D. W. Rindt indicates that microbicides are sometimes incorporated into the coating to prolong microbial degradation of the coating and to thereby extend the effective life of the fertilizer. Specifically, U.S. Pat. No. 3,342,577 demonstrated the necessity of including a microbicide in a soft wax coating to prevent its rapid microbial degradation in soil.
In U.S. Pat. No. 3,295,950 to G. M. Blouin and D. W. Rindt, which is assigned to TVA, synergism in controlled release properties is demonstrated for a two layer coating of sulfur and soft wax. The soft wax layer consists typically of petrolatum or petroleum soft waxes or oils. This synergism is demonstrated by showing that, as measured by dissolution in water, the wax coating alone provided no controlled release properties but dramatically improved the controlled release properties of SCU.
The prior art has also disclosed the optional addition to sulfur of materials such as polysulfide plasticizers, micronutrients or fillers such as vermiculite. The prior art (U.S. Pat. No. 3,295,950) has further described the benefits from a soft wax precoat. Furthermore, the prior art has disclosed that the use of a soft wax top coating has required addition of a conditioner such as diatomaceous earth or clay to the surface of the particle to render the product free flowing and for product handling and storage as described by Rindt, Blouin and Getsinger (J. Agric. & Food Chem., 16, 773, September/October, 1968). Typical products of this type have included about 2-3% sealant and about 2-2.5% conditioner.
The seven day dissolution rate is a test of the amount of urea nitrogen which dissolves when a 50 gram sample of the product is immersed in 250 ml of water at 37.8.degree. C. (100.degree. F.) for seven days. From such testing, it has been determined that the seven day dissolution rate of the prior art TVA-type coated SCU products including about 2-3% sealant and about 2-2.5% conditioner was in the range of 15-35%. In accordance with guidelines published by the Association of American Plant Food Control Officials (AAPFCO), for a coated fertilizer to be classified as a controlled release ("slow release") product, it must contain a minimum of 15% coated slow release nitrogen as measured by AOAC Official Method of Analysis (1990) number 970.04. Typically, samples that demonstrate less than 50% seven day dissolution rate in static water will demonstrate considerable slow release nitrogen by the test method 970.04. Thus, the prior art coatings have been considered to be "controlled release" by the AAPFCO guidelines. However, it is commonly known that water dissolution rate alone does not relate well to the performance of coated fertilizers under field conditions (see TVA Publication Y-181 and Journal AOAC Vol. 68, No. 4).
In this regard, it should be noted that testing has indicated that an SCU product having a top coating of SAE 30 oil should be classified in accordance with its leach test data to have excellent controlled release properties. However, when this product was imbedded in soil, capillary action removed the oil and caused rapid dissolution of the fertilizer. Similarly, certain waxes provide a top coat that greatly enhances controlled release properties as demonstrated by the water dissolution tests. However, in the soil, these top coats are quickly degraded by microbes and much of the added controlled release is lost (see "Sulfur Coating on Nitrogen Fertilizer to Reduce Dissolution Rate," D. W. Rindt; G. M. Blouin; and J. G. Getsinger; Agricultural and Food Chemistry, Vol. 16, No. 5, p. 773, September, 1968).
As a further example, a sulfur only SCU product can be given dramatically improved slow release nitrogen ratings as assessed by AOAC Method 970.04 and seven day dissolution values merely by increasing the coating weight of sulfur (e.g., from 17% to 25% sulfur). Field agronomic studies have shown, however, that the 25% coating does not provide additional slow release benefits. Such coating simply decreases the efficiency of the SCU by contributing to "lock off", the portion of the nitrogen which does not release within a growing season. Therefore, it can be seen that although fertilizers may be classifiable as "slow release" through the AAPFCO definition, the only manner of accurately assessing the true agronomic value of such fertilizer is through agronomic testing.
In summary, the prior art has disclosed multilayer coatings in which sulfur is the first layer and a secondary coating such as a soft wax and/or an oil/polyethylene (e.g., a brightstock oil/polyethylene) is applied to seal the flaws and imperfections in the sulfur coating. Such coatings provide a potent barrier to moisture and essentially prevent the immediate release normally encountered with sulfur-only coatings. The presence of these coatings also enables some reduction in the thickness of the sulfur coat required thereby reducing the undesireable "lock off" effects (i.e., the fertilizer becomes agronomically ineffective as a result of the inability of the product to release nutrient values during a given fertilization period). However, the prior sulfur coated products still suffer from a significant defect in regard to their ability to provide acceptable release characteristics over extended periods of time.
Specifically, prior sulfur coated fertilizers including topcoated sulfur coated urea products have almost exclusively exhibited release mechanisms which fit the model of a "matrix kinetic" pattern of release of encapsulated materials as characterized and discussed in the literature (See Patrick B. Deasy, "Microencapsulation and Related Drug Processes," Marcel Dekker, Inc., New York, 1984, at pages 311-316). For example, as previously noted, prior topcoated SCU products have released their nutrient values based on the microbial degradation of the sealant coating in order to allow the encapsulated fertilizer to release from the core through the defects in sulfur coating. Such a release pattern is in accordance with matrix kinetics wherein the release of the encapsulant proceeds at a rate linear to the square root of time. Thus, these products have demonstrated release patterns providing a rapid initial release of active ingredients followed by a rate of release which diminishes over time. Release patterns of this type have been recognized to significantly limit the usefulness of such products as extended residual, controlled release fertilizers.
In U.S. Pat. No. 3,576,613, an agronomically improved sulfur coated fertilizer was disclosed which included a finely divided powdered subcoat applied directly to the fertilizer core beneath a sulfur coating. A hydrophobic sealant topcoat was also disclosed for application over the powder-subcoated, sulfur-encapsulated fertilizers. However, although such products have demonstrated certain improvements in release characteristics, they exhibit the disadvantage of requiring an additional coating step in the process which adds significantly to the complexity of the production process and the cost of the resulting products.
Exemplary of particularly preferred topcoated sulfur coated fertilizer products are those polymer coated products described in commonly assigned, co-pending U.S. patent application Ser. No. 07/655,157, filed Feb. 14, 1991, entitled "Abrasion Resistant Coatings for Fertilizers," the disclosure of which is incorporated herein by reference. However, agronomic testing of the products disclosed in that application demonstrates that such products function on the basis of matrix kinetic patterns of release and, accordingly, suffer from the same disadvantages as discussed above in regard to their inability to provide extended residuals over long periods of time.
In contrast to the matrix kinetic release patterns of sulfur coated products, including polymer topcoated sulfur coated ureas, polymer coated fertilizers such as those described in commonly assigned, U.S. Pat. No. 5,089,041, entitled "Encapsulated Slow Release Fertilizers," the disclosure of which is also incorporated herein by reference and those sold by Sierra Chemical Co. under the trade name "Osmocote" have been known which release their nutrient core primarily via a diffusion mechanism. In such products, the release of nutrients from coated prills is initiated by movement of water vapor through the coating which dissolves the soluble core; the nutrients in solution then diffuse outwardly through the coating membrane and into the soil (Janick, J., "Horticultural Reviews," Vol. 1, p. 89).
Products which demonstrate diffusion kinetic release mechanisms are known as reservoir devices and such products provide a uniform release which does not taper off with time. That is, such products demonstrate release of nutrients such as fertilizers from a coated or encapsulated core at a diffusion controlled rate of essentially zero-order such that the core material releases from the coating essentially linearly over time. Thus, these products provide longer residual nutrient application characteristics over a longer period of time with increased efficiency of feeding of turf and other crops.
The primary disadvantage of such prior polymer or resin coated products providing diffusion controlled release is one of cost of production resulting from the amount of relatively expensive polymeric coating material required in order to achieve the desired release capabilities. Also, the processing equipment including the recovery systems as well as the very precise application equipment and techniques required to produce such products have added significantly to their cost of production.